The present invention relates to the art of forming structural components by use of high pressure fluid, and more particularly to a method of forming a tubular blank into a structural component by use of high pressure fluid. The application of the method of forming a tubular blank into a structural component is primarily directed toward the production of structural components of the type used in the automotive field, and it will be described in this invention with particular reference thereto; however, the invention has much broader applications and may be used to form various structural components from metal blanks for use in many other industries (e.g. aeronautics industry, shipping industry, chemical and petroleum industry, biomedical industry, etc.).
In the past, metal structural components were normally produced by stamping, forming and welding. In an effort to obtain complex shapes, metal components have been formed by a hydroforming process using metal tubular blanks formed of sheet steel material having specific initial strength and elongation properties. The metal tubular blank was cut to length and pre-bent or preformed into a shape approximating the shape of the finished structural component. The preformed metal tubular element was then loaded into a two-piece die and closed in a hydraulic press typically having a closing pressure between about 3500–8000 tons. The exposed ends of the metal tubular blank were sealed, and the metal tubular blank was then filled with a water and oil mixture. The internal pressure of the water and oil mixture inside the metal tubular blank was raised to a high level in the general neighborhood of 20,000–80,000 psi, which pressurized liquid expanded the metal tubular blank into the shape of the steel die cavity formed in the two die members of the die set carried by the hydraulic press. The cavities of the two die members have the desired final shape for the structural component so that as the metal tubular blank was expanded into the cavity, the outer shape of the component captured the shape of the cavity. This process produced a relatively accurate complex outer shape for the structural component. To relieve the fluid pressure in the formed structural component, holes were pierced into the formed structural component. Thereafter, the two die members were opened by the hydraulic press and the liquid was drained from the formed structural component. Secondary machinery operations, such as trimming and cutting mounting holes, were then performed to produce a desired component for final assembly.
This process for forming a metal tubular blank has gained in popularity because it forms the final structural component from the inside so complex shapes are possible; however, the total cycle time for hydroforming is at least about 25–45 seconds. The equipment to direct high pressure liquid into the metal tubular blank is extremely large and expensive. In addition, the die members are expensive machined parts that have a relatively short life. Hydroforming operations have a general limitation in that the process is used primarily to bend of the tubular blank, since the metal being formed is processed at ambient temperature which limits the maximum strain rate for the formed metal. The pressure of the liquid used in the hydroforming must be extremely high to deform the relatively cold sheet metal of the tubular blank into simple configurations. Consequently, hydroforming is used primarily for bending and straightening metal tubular elements into the desired final shape. Even though there are process limitations in using hydroforming to make metal tubular structural components, a substantial technology field has developed around this process. In one feature of hydroforming, the sheet steel tubular blank is formed into a desired shape while additional metal material is forced axially into the die cavity so that the wall thickness of the formed structural component does not drastically decrease as the volume of a given cross section increases during the processing by high pressure liquid.
Hydroforming is the primary prior art constituting the background of the present invention. However, blow forming of plastic sheets has been used for years to produce high volume plastic containers using conventional steel die members. Of course, such die members used in plastic blow forming cannot be used for forming steel. For that reason, hydroforming is used for metal instead of blow forming which is principally used in the plastics industry. The highly developed technologies of hydroforming of steel tubes and blow forming of plastic sheets constitute the background of the present invention; however, these two forming processes are not economically usable for forming sheet steel tubular blanks into tubular structural components. In addition, these two processes do not have the capability of controlling the metallurgical characteristics along the length of the metal tubular blank, as obtainable by the present invention.
Although hydroforming of sheet steel and blow forming of plastic sheets constitute the principal background material to the present invention, it has been found that certain features of the technology disclosed by Boeing Company in the patents identified above for superplastic forming sheet metal plates by high pressure gas can be used in practicing the invention. The Boeing Company processes are not background information from the standpoint that such processes are not capable of forming a shaped metal blank into a structural tubular component and are not capable of controlling the metallurgical characteristics of the metal forming the structural tubular component.
In view of the prior art, there is a need for a process for forming metal tubular blanks into simple or complex shapes which process is more economical that past processes, which process is less complex that past processes, which process has extended life for the forming components used to form the metal structural blanks, which process can quickly form metal structural blanks into various shapes, and which process is capable of controlling the metallurgical characteristics of the metal forming the structural tubular component.